The first practical large scale application of
stratigraphy was by William
Smith in the 1790s and early 1800s. Smith, known as the Father
of English Geology, created the first geologic map
of England, and first recognized the significance of strata or rock
layering, and the importance of fossil markers for correlating
strata. Another influential application of stratigraphy in the
early 1800s was a study by Georges
Cuvier and Alexandre
Brongniart of the geology of the region around Paris.

Lithologic stratigraphy

Lithostratigraphy, or lithologic stratigraphy, is
the most obvious. It deals with the physical lithologic or rock type change
both vertically in layering or bedding of varying rock type and
laterally reflecting changing environments of deposition, known as
facies change. Key
elements of stratigraphy involve understanding how certain
geometric relationships between rock layers arise and what these
geometries mean in terms of depositional environment. One of
stratigraphy's basic concepts is codified in the Law of
Superposition, which simply states that, in an undeformed
stratigraphic sequence, the oldest strata occur at the base of the
sequence. Chemostratigraphy is based on the changes in the relative
proportions of trace elements and isotopes within and between
lithologic units. Carbon and oxygen isotope ratios vary with
time and are used to map subtle changes in the paleoenvironment This
has led to the specialized field of isotopic stratigraphy.

Cyclostratigraphy documents the often cyclic
changes in the relative proportions of minerals, particularly carbonates, and fossil
diversity with time, related to changes in palaeoclimates.

Biostratigraphy

Biostratigraphy or paleontologic stratigraphy
is based on fossil
evidence in the rock layers. Strata from widespread locations
containing the same fossil fauna and flora are correlatable in
time. Biologic stratigraphy was based on William Smith's
principle of faunal succession, which predated, and was one of
the first and most powerful lines of evidence for, biological
evolution. It provides strong evidence for formation (speciation) of and the
extinction of
species. The geologic
time scale was developed during the 1800s based on the evidence
of biologic stratigraphy and faunal succession. This timescale
remained a relative scale until the development of radiometric
dating, which gave it and the stratigraphy it was based on an
absolute time framework, leading to the development of
chronostratigraphy.

One important development is the Vail curve,
which attempts to define a global historical sea-level curve
according to inferences from world-wide stratigraphic patterns.
Stratigraphy is also commonly used to delineate the nature and
extent of hydrocarbon-bearing
reservoir rocks, seals and traps in petroleum
geology.

Chronostratigraphy

Chronostratigraphy is the branch of
stratigraphy that studies the absolute age of rock strata.

Chronostratigraphy is based upon deriving
geochronological
data for rock units, both directly and by inference, so that a
sequence of time relative events of rocks within a region can be
derived. In essence, chronostratigraphy seeks to understand the
geologic history of rocks and regions.

The ultimate aim of chronostratigraphy is to
arrange the sequence of deposition and the time of deposition of
all rocks within a geological region, and eventually, the entire
geologic record of the Earth.

Magnetostratigraphy

When measurable magnetic properties of
rocks vary stratigraphically they may be the bases for related but
different kinds of stratigraphic units known collectively as
"magnetostratigraphic units" ("magnetozones"). The magnetic
property most useful in stratigraphic work is the change in the
direction of the remanent magnetization of the rocks, caused by
reversals in the polarity of the Earth's magnetic field. Such
reversals of the polarity have taken place many times during
geologic history. They are recorded in the rocks because the rocks
may record the direction of the Earth's magnetic field at or near
the time of rock formation (see paleomagnetism). The
direction of the remnant magnetic polarity recorded in the
stratigraphic sequence can be used as the basis for the subdivision
of the sequence into units characterized by their magnetic
polarity. Such units are called "magnetostratigraphic polarity
units".

Magnetostratigraphy is a chronostratigraphic
technique used to date sedimentary and volcanic stratigraphic
sections. The method works by collecting oriented samples at
measured intervals throughout the section. The samples are analyzed
to determine their Detrital Remanent Magnetization (DRM), that is,
the polarity of Earth's magnetic field at the time a stratum was
deposited. This is possible because when very fine-grained magnetic
minerals (< 17 micrometres) fall through the water column, they
orient themselves with Earth's magnetic field. Upon burial, that
orientation is preserved. The minerals, in effect, behave like tiny
compasses.

If the ancient magnetic field was oriented
similar to today's field (North Magnetic Pole near the North
Rotational Pole) the strata retain a Normal Polarity. If the data
indicate that the North Magnetic Pole was near the South Rotational
Pole, the strata exhibit Reversed Polarity.

Sampling procedures

Oriented paleomagnetic core samples are
collected in the field using a Pomeroy Drill. A minimum of three
samples is taken from each sample site for statistical purposes.
Spacing of the sample sites within a stratigraphic section depends
on: 1) the type of depositional environment: The farther away from
the orogenic front, the closer the sample spacing due to generally
lower rates of deposition; and 2) the suitability of the rocks for
paleomagnetic analysis. Mudstones, siltstones, and very
fine-grained sandstones are the preferred lithologies because the
magnetic grains are finer and more likely to orient with the
ambient field during deposition. It is more likely that these
samples will deliver a reliable paleomagnetic signal.

Analytical procedures

Samples are first analyzed in their
natural state to obtain their
Natural Remanent Magnetization (NRM). The NRM is then stripped
away in a stepwise manner using thermal or alternating field
demagnetization techniques to reveal the stable magnetic component.
The stable component is usually interpreted to be the DRM.

DRM orientations of all samples from a site are
then compared and their magnetic polarity is determined with Fisher
statistics. Using Watson's criteria, the statistical significance
of each sample site is evaluated. The latitudes of the Virtual
Geomagnetic Poles from those sites determined to be statistically
significant are plotted against the stratigraphic level at which
they were collected. These data are then abstracted to the standard
black and white magnetostratigraphic columns in which black
indicates Normal polarity and white is Reversed polarity.

Correlation and ages

Because the polarity of a stratum can
only be Normal or Reversed, variations in the rate at which the
sediment accumulated can cause the thickness of a given polarity
zone to vary from one area to another. This presents the problem of
how to differentiate different zones of like polarities between
different stratigraphic sections. To overcome the possibility of
confusion at least one isotopic age (or at least an age based on
paleontological data) needs to be collected from each section.
These are usually obtained from intercalated airfall volcanic
material. With the aid of the independent isotopic age or ages, the
local magnetostratigraphic column is correlated with the Global
Magnetic Polarity Time Scale (GMPTS).

Because the age of each reversal shown on the
GMPTS is relatively well known, the correlation establishes
numerous time lines through the stratigraphic section. These ages
provide relatively precise dates for features in the rocks such as
fossils, changes in sedimentary rock composition, changes in
depositional environment, etc. They also constrain the ages of
cross-cutting features such as faults, dikes, and unconformities.

Sediment accumulation rates

Perhaps the most powerful application of these
data is to determine the rate at which the sediment accumulated.
This is accomplished by plotting the age of each reversal (in
millions of years ago) vs. the stratigraphic level at which the
reversal is found (in meters). This provides the rate in meters per
million years which is usually rewritten in terms of millimeters
per year (which is the same as kilometers per million years).

These data are also used to model basin
subsidence rates. Knowing the depth of a hydrocarbon source rock
beneath the basin-filling strata allows calculation of the age at
which the source rock passed through the generation window and
hydrocarbon migration began. Because the ages of cross-cutting
trapping structures can usually be determined from
magnetostratigraphic data, a comparison of these ages will assist
reservoir geologists in their determination of whether or not a
play is likely in a given trap.

Another application of these results derives from
the fact that they illustrate when sediment accumulation rates
changed. Such changes require explanation. The answer is often
related to either climatic factors or to tectonic developments in
nearby or distant mountain ranges. Evidence to strengthen this
interpretation can often be found by looking for subtle changes in
the composition of the rocks in the section. Changes in sandstone
composition are often used for this type of interpretation.

Archaeological stratigraphy

In the field of archaeology, soil
stratigraphy is used to better understand the processes that form
and protect archaeological
sites. The law of superposition holds true, and this can help
date finds or features from each context, as
they can be placed in sequence and the dates interpolated. Phases
of activity can also often be seen through stratigraphy, especially
when a trench or feature is viewed in section
(profile). As pits and other features can be dug down into earlier
levels, not all material at the same absolute depth is necessarily
of the same age, but close attention has to be paid to the
archeological layers. The
Harris-matrix
is a tool to depict complex stratigraphic relations, as they are
found, for example, in the contexts of urban
archaeology.